In order to realize a highly efficient operation in an amplifier, it is ideal to use the amplifier near saturated electric power and optimize load impedance taking into account even harmonics such as a second harmonics and a third harmonics. Amplifiers are called class-A, class-B, class-C, class-J, class-D, class-E, and class-F according to the load impedance and overdrive and depths of bias. However, these are classifications for convenience. Essential operations are similar. In particular, the amplifiers have similar dynamic load lines under a condition in which maximum efficiency is obtained. That is, in the case of a common source, an operation condition is set to minimize power consumption inside a device such that time waveforms of a drain current and a drain voltage (in the case of bipolar, a collector current and a collector voltage when a common emitter) do not overlap at the same time, that is, an Instantaneous product of an electric current and a voltage does not increase.
In general, as means for realizing any load impedance, a resonant no loss matching (tuned) circuit is adopted. Therefore, a range of frequencies in which target impedance is obtained is limited to a tuning frequency range. It is difficult to independently control impedance between adjacent harmonics such as between a DC and a fundamental frequency, between the fundamental frequency and a second harmonics, and between the second harmonics and third harmonics. It is necessary to improve frequency separation using a circuit having high Q to show ideal impedances at respective frequencies. As a result, a band is narrowed. In this way, high efficiency and a wide band is in a contradictory relation. Therefore, conventionally, in general, a high-efficiency circuit has a narrow band.
As an amplifier according to a first related technique, there is known an amplifier that, making use of a fact that the Impedance of a series resonator composed of a capacitor (a capacitor X1) and a coil (an inductor) (an Inductor Y1) is zero at fundamental frequency, gives load impedance at the fundamental frequency using a series circuit of an inductor Y1′ connected in series to the series resonator and a load resistor and parallel connection of the series circuit and a capacitor (a capacitor X2). In the amplifier, only the capacitor X2 is shown making use of the fact that series impedance of the capacitor X1 and the Inductor Y1 is high with respect to harmonics. An inductor (an inductor Y2) parallel to the capacitor X2 is selected to allow only a direct current (DC) to pass and have high impedance at a radio frequency (RF). A resonant frequency of a series resonator composed of Y1-Y1′-X1 is lower than the fundamental frequency. If a Q value of a series resonator composed of the capacitor X1 and the inductor Y1 is set to be low, even if the resonant frequency is detuned from the fundamental frequency, a value close to satisfactory load impedance (or load admittance) can be maintained. However, an ideal operation cannot be obtained because harmonic impedance decreases.
As another form of the amplifier, a switching operation performed by interchanging a relation between the capacitor X2 and the inductor (the coil) Y1′ has been examined (a second related technique). It has been demonstrated that equivalent characteristics can be obtained by appropriately selecting load impedance.
In the first related technique, it is known that an angle of load impedance in the fundamental frequency is approximately 33 degrees and an angle given by the inductor Y1′ is approximately 49 degrees. There is known a method of giving, with another means, the angle given by the inductor Y1′ and further connecting the capacitor X2 to optimize a circuit such that the angle is fixed at approximately 33 degrees within a desired band (a third related technique). In the third related technique, it has been demonstrated that the angle of the load impedance can be kept fixed in a band by connecting an inductor Y″ In parallel to the parallel capacitor X2 instead of the inductor Y1′ connected in series to the series resonator. In the inductor Y″, the Inductor Y2 in the first related technique is set to a finite value. However, the third related technique demonstrates that susceptance of the inductor Y″ is cancelled by susceptance of the series resonant circuit of X1-Y1 to obtain a fixed angle of 49 degree in a band. If the parallel capacitor X2 is connected to the inductor Y″, an angle in the band inclines. A method for circuit optimization by a circuit analysis CAD is used in order to fix the load impedance at 33 degrees. Eventually, it has not been clarified by what kind of mechanism the load impedance with the fixed angle is realized. Therefore, theory has not reached calculation of Y″ by algebraic calculation. A load circuit equivalent to X1-Y1 is replaced with a n impedance converter and a T Chebyshev band-pass filter. However, since a Q value of the entire circuit is low, harmonic impedance greatly deviates from an ideal value, power efficiency and frequency characteristics of output power are poor, and the number of elements of the circuit is large.
As another form of the amplifier of the first related technique, there is an attempt to reduce the inductance of the inductor Y2 and use, as a parallel resonator, a circuit composed of the inductor Y2 and the inductor X2 (a fourth related technique). Unlike the third related technique, Y2 (equivalent to Y″ of the third related technique) is algebraically calculated. As a result, it has been demonstrated that there is possibility of presence of a design condition in which an angle that should be shown at an operation frequency of the series resonator composed of Y1-Y1′-X1 can be reduced to zero (that is, Y′ can be deleted) by setting Y2 to a finite value. When Y2 is set to the finite value and Y′ is deleted, a change to a circuit topology different from a circuit topology in the first related technique is enabled. That is, it is possible to configure a highly efficient amplifier with the parallel resonator by X2-Y2 (or X2-Y″) and the series resonator by X1-Y1.
On the other hand, a method of compensating for reactance (equivalent to the angle mentioned above) of the parallel resonator has been known since long time ago. For example, there is known a method of connecting the series resonator to the parallel resonator or connecting the series resonator to the parallel resonator and further connecting the parallel resonator to compensate for a reactance component (an imaginary part of impedance; equivalent to the angle mentioned above) (a fifth related technique). A principle of this method is simple. This makes use of the fact that, in the principle of an electric circuit, whereas the reactance component of the parallel resonator decreases near a resonant point, the reactance of the series resonator increases near the resonant point. The increase and the decrease in the reactances are offset and values of the reactances generally coincide. Inclinations of the reactances are adjusted by Q values of the respective resonators. The fifth related technique is a trial itself in the third related technique.
There is known a sixth related technique in which the fifth related technique is applied to the amplifier of the fourth related technique and a design condition unclear in the third related technique is clarified. However, in the derived design condition, it is necessary to use a value around 1 as the Q value of the resonator. Therefore, even if the reactance component of the fundamental frequency can be compensated, a real part of admittance or impedance fluctuates in the fundamental frequency band. In addition, a real part of admittance does not sufficiently decrease in a frequency band of a second harmonics. As a result, satisfactory characteristics cannot be obtained in a wide band. In this regard, a technical progress from the third related technique can be considered to be absent.
There has been a problem in that compensation of the imaginary part (the reactance component) of the impedance and the imaginary part (the susceptance component) of the admittance is focused and it is not taken into account at all that flatness of the real part necessary for fixing output power in the fundamental frequency band and a sufficiently low real part in a harmonic band necessary for efficiency improvement, voltage amplitude suppression, and harmonic leak prevention are realized. As it is seen in, for example, the third related technique, it has been necessary to connect, to a post stage, in addition to a reactance compensation circuit, a post-stage circuit for converting optimum impedance of the fundamental frequency to 50Ω in order to connect the optimum impedance to an external circuit and it has been necessary to connect, to the post stage, a wideband cutoff filter for cutting off a leak of harmonics. Therefore, a circuit size is considerably large.